1. Field of the Invention
The present invention relates to an automatic frequency controller applied to digital demodulation processing in satellite communications, mobile satellite communications and mobile land communications, and a demodulator unit for demodulating a signal whose frequency is controlled by the automatic frequency controller.
2. Description of Related Art
Recently, intensive researches of digital modulation and demodulation have been conducted in the field of the satellite communications, mobile satellite communications and mobile land communications. In the environment of mobile communications, in particular, communication signals undergo fading. Hence, various demodulation schemes have been proposed which can achieve stable operation in such a fading environment.
A scheme which has attracted attention makes absolute coherent detection possible even under a fading environment by estimating and compensating for fading distortion using a known signal. To apply the scheme, the fading distortion is estimated and compensated for after carrying out quasi-coherent detection. In this case, it is necessary for achieving highly accurate estimation and compensation of the fading distortion to reduce the frequency offset between the carrier frequency of a received communication signal and the oscillation frequency of a reference signal for the quasi-coherent detection.
However, when only insufficient frequency stability and accuracy can be obtained of an oscillation circuit of a transmitter or receiver, a problem arises that the highly accurate estimation and compensation of the fading distortion cannot be achieved without eliminating the frequency offset by carrying out some processing.
In addition, in mobile communications, transmission and reception are carried out between a fixed station and mobile stations, or between mobile stations. Accordingly, when two stations are moving relatively, the frequency shift of a transmitted signal occurs because of Doppler fluctuations. This causes the frequency offset between the carrier frequency of the received communication signal and the oscillation frequency of the reference signal in spite of the high frequency stability and accuracy of the oscillation circuit of the transmitter and receiver.
A technique for compensating for the frequency offset is disclosed in xe2x80x9cFrequency Offset Compensation Method for QAM in Land Mobile Communicationsxe2x80x9d, by Kato and Sasaoka, the Transaction of the Institute of Electronics, Information and Communication Engineers of Japan, (B-II), Vol. J74-B-II, No.9, pp.493-496 (September. 1991). The technique disclosed in the prior art document eliminates the frequency offset using the phase fluctuation information of a known signal (pilot signal).
In the conventional technique, a transmitting side inserts into a communication signal a known signal consisting of one symbol at every insertion period NF. On the other hand, a receiving side calculates a phase variation quantity between two different known signals, and rotates the phase of the received signal in accordance with the phase variation quantity calculated, thus eliminating the frequency offset.
In the conventional technique, the estimation range and estimation accuracy of the frequency offset are determined by the insertion period NF of the known signal. More specifically, considering that the detection range of the phase variation quantity xcex94xcex8 between the known signals is xe2x88x92xcfx80xe2x89xa6xcex94xcex8xe2x89xa6xcfx80, the frequency offset estimation range fDET falls in the range from equal to or greater than xe2x88x92R8/2NF (Hz) to equal to or less than Rs/2NF (Hz) as expressed by the following equation (1).                               f                      D            ⁢                          xe2x80x83                        ⁢            E            ⁢                          xe2x80x83                        ⁢            T                          =                              -                                                            R                  ⁢                                      xe2x80x83                                                  S                                            2                ⁢                                  N                  F                                                              ⁢                                    (              Hz              )                        ~                                          R                S                                            2                ⁢                                  N                  F                                                              ⁢                      (            Hz            )                                              (        1        )            
where Rs is a transmission rate of the communication signal.
On the other hand, improvement in the frequency offset estimation accuracy has been desired recently. To achieve this, the effect of noise must be eliminated, which can be carried out by narrowing the frequency offset estimation range fDET. To implement this, the insertion period NF must be increased as clearly seen from equation (1). However, when the insertion period NF of the known signal is fixed by communication protocols, it cannot be changed, in which the improvement in the frequency offset estimation accuracy is difficult.
Furthermore, the conventional technique does not have any structure for changing the object to be subjected to the frequency offset estimation in response to the state of transmission paths. This presents another problem of making it difficult to sufficiently improve the demodulation quality. In an environment like a so-called Ricean fading environment, in which a direct wave experiencing the Doppler fluctuations is mixed with multipath waves, the improvement in the demodulation quality can be achieved more effectively by identifying a received wave with stronger power and by estimating and compensating for the frequency offset of the received wave.
Therefore, an object of the present invention is to provide, by solving the foregoing technical problems, an automatic frequency controller that can improve frequency offset estimation accuracy without changing the insertion period of the known signal, and carry out elimination processing of the frequency offset in response to the state of transmission paths.
Another object of the present invention is to provide a demodulator unit with improved demodulation accuracy by using the automatic frequency controller.
To accomplish the objects of the present invention, a frequency offset estimating section is provided for estimating a frequency offset of a received signal including periodic known signals. The frequency offset estimating section obtains the phase fluctuation quantity of each of the plurality of known signals included in the received signal.
The frequency offset estimating section further obtains signal powers corresponding to a plurality of candidate frequency offsets set at predetermined frequency offset estimation accuracy intervals in a frequency offset estimation range that is determined by the insertion period of the known signal.
The frequency offset estimating section sums up, from among the power signals obtained, the signal powers of candidate frequency offsets included in a frequency window with a predetermined frequency width, thereby obtaining window power corresponding to any one of the candidate frequency offsets within the frequency window.
Specifically, the frequency width of the frequency window is set in accordance with the fading state of a transmission path studied in advance, for example. More specifically, the frequency width is set relatively narrow on a transmission path resembling a Gaussian channel in which the power of a direct wave experiencing the Doppler fluctuations is relatively large. In contrast, the frequency width is set relatively wide on a transmission path resembling a Rayleigh fading channel in which the power of multipath waves is relatively large.
The signal power at the frequency of the direct wave can be maximized by summing up the signal powers in each frequency window with the relatively narrow frequency bandwidth, and by associating the window power with the candidate frequency offset at the center of the frequency window, for example. In the opposite case, the signal power at the center frequency of the multipath waves can be maximized.
Thus, the frequency offset estimating section obtains using the frequency window the window powers corresponding to the candidate frequency offsets in the frequency offset estimation range, and detects the candidate frequency offsets corresponding to the maximum value of the window powers. Specifically, the frequency offset estimating section obtains the window powers with shifting the frequency window one by one of the frequency offset estimation accuracy unit. Furthermore, the frequency offset estimating section estimates the detected candidate frequency offsets as the frequency offsets.
This enables the frequency controlled object to be changed in accordance with the fading state of the transmission path, thereby implementing the frequency control appropriate to the fading state of the transmission path. Thus, applying the automatic frequency controller to the demodulator unit can improve the demodulation accuracy.
It is one of the candidate frequency offsets that is estimated as the frequency offset. Therefore, increasing the number of the candidate frequency offsets makes it possible to estimate the frequency offset more precisely. In this case, the number of the candidate frequency offsets in the frequency offset estimation range can be increased with maintaining the range. Thus, the estimation accuracy of the frequency offset can be improved without changing the frequency offset estimation range. As a result, the estimation accuracy of the frequency offset can be improved in an environment in which the insertion period of the known signal is defined by a protocol.